Display apparatus

ABSTRACT

A display apparatus includes a first substrate including a plurality of pixels, a first electrode arranged on the first substrate, a second substrate facing the first substrate, and a second electrode arranged on the second substrate and spaced apart from the first electrode, the second electrode to form an electric field in cooperation with the first electrode. At least one of the first and second electrodes includes a transparent conductive nanomaterial having a transmittance of no less than 73% to no more than 100% and a sheet resistance of 0 ohms to 100 ohms.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from and the benefit of Korean PatentApplication No. 2008-104728, filed on Oct. 24, 2008, which is herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus having an electrodethat includes transparent conductive nanomaterial.

2. Discussion of the Background

Recently, research and development has been conducted with various flatpanel display apparatuses, including liquid crystal displays (LCDs),organic light emitting displays (OLEDs), and plasma display panels(PDPs). These display apparatuses control a liquid crystal layer, anorganic light emitting layer, or a plasma distribution using electrodesformed on transparent substrates thereof. For example, in a liquidcrystal display including two substrates and a liquid crystal layerformed therebetween, the liquid crystal layer includes liquid crystalmolecules having optical anisotropic property, which are driven by anelectric field. The liquid crystal molecules are disposed between twoelectrodes forming the electric field and aligned in a predetermineddirection according to the electric field, thereby changing the lighttransmittance thereof and displaying images.

Indium tin oxide (ITO) and indium zinc oxide (IZO) may be used asmaterials in the display panel electrode. However, in order to form theITO or IZO electrode, a deposition process performed at high temperaturemay be required and the physical properties of these materials may bedifficult to predict. In addition, ITO and IZO may exert influences onother structures of the display panel due to oxygen atoms thereof andmay be vulnerable to a wet etch process.

SUMMARY OF THE INVENTION

The present invention provides a display apparatus having improved sheetresistance and transmittance.

The present invention also provides a thin film transistor (TFT)substrate having a transparent conductive electrode including nanowiresor metal oxide nanoparticles.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

The present invention discloses a display apparatus including a firstsubstrate including a plurality of pixels, a first electrode arranged onthe first substrate, a second substrate facing the first substrate, anda second electrode arranged on the second substrate and spaced apartfrom the first electrode, the second electrode to form an electric fieldin cooperation with the first electrode. At least one of the firstelectrode and the second electrode includes a transparent conductivenanomaterial having transmittance of no less than 73% to no more than100%, and sheet resistance of 0 ohms per square (Ω/□) to 100 ohms persquare.

The present invention also discloses a thin film transistor (TFT)substrate. The thin film transistor substrate includes a TFT arranged ona substrate and a transparent conductive electrode connected to the TFT.The transparent conductive electrode includes nanowires having a densityof 4 particles to 40 particles per 5×5 square micrometer.

The present invention also discloses a TFT substrate. The TFT substrateincludes a TFT arranged on a substrate and a transparent conductiveelectrode connected to the TFT. The transparent conductive electrodeincludes metal oxide nanoparticles having a density of 400 particles to3000 particles per 5×5 square micrometer.

The present invention also discloses a TFT substrate. The TFT substrateincludes a TFT arranged on a substrate and a transparent conductiveelectrode connected to the TFT. The transparent conductive electrodeincludes carbon nanotubes having a density of 4 particles to 150particles per 5×5 square micrometer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, show exemplary embodiments of the invention,and together with the description serve to explain the principles of theinvention.

FIG. 1 is a plan view showing a part of a display panel according to anexemplary embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II′ of FIG. 1.

FIG. 3A and FIG. 3B are sectional views showing vertical and lateralelectric fields of a display panel, respectively.

FIG. 4 is a graph showing transmittance of nanowires in relation to agray scale voltage applied to a display panel having a cell gap of about5 μm.

FIG. 5 is a graph showing transmittance of metal oxide nanoparticles inrelation to a gray scale voltage applied to a display panel having acell gap of about 5 μm.

FIG. 6 is a graph showing transmittance of carbon nanotubes in relationto a gray scale voltage applied to a display panel having a cell gap ofabout 5 μm.

FIG. 7A is a photograph showing gold-silver nanoparticles as arepresentative nanomaterial.

FIG. 7B is a photograph showing gold-silver nanocomplex of thegold-silver nanoparticles.

FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, and FIG. 8E are sectional viewsshowing a method of manufacturing a display apparatus according to anexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which exemplary embodiments of the inventionare shown. This invention may, however, be embodied in many differentforms and should not be construed as limited to the exemplaryembodiments set forth herein. Rather, these exemplary embodiments areprovided so that this disclosure is thorough, and will fully convey thescope of the invention to those skilled in the art. In the drawings, thesize and relative sizes of layers and regions may be exaggerated forclarity. Like reference numerals in the drawings denote like elements.

It will be understood that when an element or layer is referred to asbeing “on” or “connected to” another element or layer, it can bedirectly on or directly connected to the other element or layer, orintervening elements or layers may be present. In contrast, when anelement is referred to as being “directly on” or “directly connected to”another element or layer, there are no intervening elements or layerspresent.

FIG. 1 is a plan view showing a part of a display panel according to anexemplary embodiment of the present invention. FIG. 2 is a sectionalview taken along line II-II′ of FIG. 1. Although not shown in FIG. 1 andFIG. 2, the display panel includes a plurality of pixels defined by aplurality of gate lines and a plurality of data lines crossing the gatelines. However, in the present exemplary embodiment, for the convenienceof explanation, one pixel will be described as a representative example.

Referring to FIG. 1 and FIG. 2, a display panel 100 includes a firstsubstrate 110 and a second substrate 130 facing the first substrate 110.In addition, the display panel 100 may further include a liquid crystallayer 150 (or an electrophoretic layer) disposed between the firstsubstrate 110 and the second substrate 130 to display an image.

The liquid crystal layer 150 includes liquid crystal molecules, whichhave an optical anisotropy and are aligned in a certain direction inresponse to a voltage applied thereto, so that desired images aredisplayed corresponding to the light transmittance of the liquid crystalmolecules with respect to light traveling through the liquid crystallayer 150.

In comparison, the electrophoretic layer includes electrophoreticparticles, which are dispersed in a liquid and have a charge on theirsurface. The electrophoretic particles move to a certain area in theliquid in response to a voltage applied thereto, thereby displaying animage.

In the present exemplary embodiment, the display panel 100 will bedescribed on the assumption that the material disposed between the firstand second substrates 110 and 130 is the liquid crystal layer 150.However, the scope of the present invention is not limited thereto. Thatis, the material disposed between the first and second substrates 110and 130 may be the electrophoretic layer.

The first substrate 110 may be referred to as a TFT substrate when a TFTis formed on the first substrate 110, or may be referred to as a lowersubstrate when the first substrate is located at a relatively lowerposition. The second substrate 130 may be referred to as a color filtersubstrate when a color filter is formed on the second substrate, or maybe referred to as an upper substrate when the second substrate islocated at a relatively upper position.

The first substrate 110 includes a first insulating substrate 101. Thefirst insulating substrate 101 may include a transparent insulatingmaterial such as glass, quartz, plastic, etc.

A plurality of gate lines 111 and a plurality of data lines 112 crossingthe gate lines 111 are formed on the first insulating substrate 101 todefine a plurality of pixel areas. A thin film transistor T is formed inthe pixel area adjacent to the gate lines 111 and the data lines 112. Apixel electrode 127 is formed in the pixel area and connected to thethin film transistor T. The pixel electrode 127 forms an electric fieldin cooperation with a common electrode 139 to drive the liquid crystallayer 150.

The thin film transistor T includes a gate electrode 113 connected to acorresponding gate line of the gate lines 111, a source electrode 121connected to a corresponding data line of the data lines 112, and adrain electrode 123 connected to the pixel electrode 127. The thin filmtransistor T further includes a gate insulating layer 115 to insulatethe gate electrode 113 from the source and drain electrodes 121 and 123,and an active layer 117 and an ohmic contact layer 119 to form aconductive channel between the source electrode 121 and the drainelectrode 123 when a gate voltage is applied to the gate electrode 113.

A protection layer 125 is formed on the first substrate 110 to cover thethin film transistor T. A contact hole 129 is formed in the protectionlayer 125 and exposes a portion of the drain electrode 123, so that thepixel electrode 127 may be connected to the drain electrode 123 throughthe contact hole 129.

The second substrate 130 is disposed to face the first substrate 110 andincludes a second insulating substrate 131. The second insulatingsubstrate 131 may include a transparent insulating material such asglass, quartz, plastic, etc.

The second substrate 130 includes a color filter 135 formed on thesecond insulating substrate 131 to display the color of each pixel. Thecommon electrode 139 is formed on the color filter 135 to form anelectric field in cooperation with the pixel electrode 127.

The display apparatus is operated by applying a common voltage to thecommon electrode 139 that serves as a reference voltage for the liquidcrystal molecules, and providing pixel signals from the data line 112 tothe pixel electrode 127 via the thin film transistor T in response toscan signals from the gate line 111, resulting in an electric fieldbeing formed between the common electrode 139 and the pixel electrode127. The liquid crystal molecules in the liquid crystal layer 150 arealigned in a certain direction by the electric field, so that the amountof light transmitted through the liquid crystal layer 150 changes,thereby displaying images. The pixel electrode 127 and the commonelectrode 139 may be referred to as a first electrode and a secondelectrode, respectively.

ITO may be used for the pixel electrode 127, the common electrode 139,or an anode of an OLED, because of its high transparency andmanufacturing experiences gained by the LCD industry. However,properties of ITO may be difficult to control in a deposition process,especially in a deposition process on a plastic substrate. The oxygenatoms in ITO may exert influences on active materials if they areremoved from the electrodes. Also, ITO may be vulnerable to a wet etchprocess, thereby restricting its yield and throughput.

Recently, a process technique in TFT manufacturing using a metal oxideelectrode, such as ITO, IZO, or a silicon semiconductor, such as Cu, Cr,Al, Nd, Mo, etc., on a glass substrate has been developed. But sincehigh temperature deposition systems or photolithography patterningschemes may be utilized for the metal oxide electrode, additionalprocesses may be required.

In addition, when a display apparatus is manufactured using a flexiblesubstrate such as plastic sheet, metal foil, paper, etc., electrodematerials may be required that can be deposited by a low-temperaturecoating process, and may have various properties such as transparency,mechanical strength, or the like. Conductive polymers,nanoparticle-based conductive inks, and carbon nanotube-based inks maybe used as the electrode materials. In this case, the electrodematerials for the display apparatus may have high mechanical strengthwhen they are bent or folded, as well as good transparency (more thanabout 70% of transmittance) with a low resistance value (about 100 Ω/□or less). The change of sheet resistance may be small or zero at highertemperature, overheating, or short-circuiting of the plastic substrate.

Moreover, the electrode materials for the display apparatus may alsohave strong chemical tolerance for various organic solvents, durabilityof heat resistance, excess moisture tolerance, etc. and fulfillstability requirements of the display apparatus. In addition, theelectrode materials may be easily patterned by various printing methodssuch as inkjet printing, gravure printing, thermal printing, rotaryprinting, or the like, to allow the electrode materials to be easilyintegrated on paper or plastics as the electrode of the TFT.

Thin metal films deposited at low temperatures, such as Ca/Ag, show lowelectrical resistance, but are difficult to print using theabove-described methods. Similarly, conductive inks of metal materialssuch as Ag, Au, which are spin-coated or inkjet printed, have shown poorperformances as compared with metal materials such as Au, Ag, etc., in abulk state. This is due to the fact that conductivity of deposited thinmetal films is less than that of the metal materials in the bulk state.The conductivity depends upon the shape of the particles of the metalmaterials, as well as their size and the material content.

Table 1 shows properties of some electrode materials. In table 1, “3”refers to excellent properties, “2” refers to good properties, and “1”refers to poor properties.

TABLE 1 dispersed carbon sput- intrinsically nanotube tered disperseddispersed conducting electrode ITO ITO Nanometal polymer transmittance 33 2 2 3 conductivity 2 3 1 3 2 cost 3 2 3 1 2 color 3 2 1 3 2printability 3 1 2 1 3 flexibility 3 1 1 3 3 stability 3 3 3 3 1

Apart from the above described characteristics of conductive electrodematerials, a film topology of the electrode materials helps determinethe optical performance of the display apparatus including suchmaterials. The film may have various morphologies such as repetitivelydistributed metal/metal-oxide micro/nano grains, matrix of welldispersed nanowires/nanoparticles of metal, metal oxide, conductivepolymers, carbon nanotubes, and so on.

Performance characteristics of an LCD may be determined through variousspecifications such as viewing angle, brightness, response time, panelsize, contrast ratio, resolution, etc. Image artifact is one of theimportant parameters that determines the overall image quality of theLCD. This is related to optical defects generated in various ways duringthe manufacturing processes for the LCD.

There are two kinds of image artifacts, spatial and temporal. Some areinherent to the technology used, while others are yield-related and canbe reduced or eliminated by optimizing the manufacturing processes.Spatial artifacts include Mura, pixel defects, and cross-talk. Temporalartifacts include image delay, flicker, and motion blur. Pixel defectsare mainly a yield issue, and cross-talk is caused by non-optimizeddriving schemes, TFT leakage currents, RC propagation delays on buslines, or capacitive coupling between pixels and bus lines.

Exemplary embodiments of present invention mainly focus on reducing theMura image artifact that denotes local differences in luminance,contrast ratio, and color performance. Mura may be caused byprocess-related defects such as liquid crystal cell-gap variation causedby non-uniform coating/rubbing of a liquid crystal alignment layer,materials-related defects such as low quality liquid crystal fluid, andmaterial/optical properties of interfacing layers such as transparentITO electrodes and passivation layers. A surface morphology of atransparent conductor is an important parameter that could affect theoptical properties of the LCD.

Liquid crystal molecules are elongated in shape and may have a length ofabout 2 nm. Because of their elongated, cigar-like shape, they tend tobe parallel to each other in the lowest energy state and thus normallyexist in bulk as microdroplets. When ITO or IZO electrodes are appliedto a conventional active matrix LCD by using a sputtering method, whenthe microdroplet-based liquid crystal molecules are sandwiched betweentwo transparent conductive electrodes spaced apart from each other by acell gap of about 5 μm, the liquid crystal molecules may be verticallyaligned according to the electric field between the two electrodes andpolarized in a specific direction.

New materials have been developed to replace for ITO or IZO used indisplays, which are based on microparticles including nanograins,nanotubes, nanowires, and so on. However, the interval between twoadjacent nanoparticles such as micrograins, nano tubes or nanowires andthe morphology of the nanoparticles exert influence on the morphologiesof films when the films are performed by spin-coating, web-coating,gravure-printing or similar techniques instead of sputtering, CVD orevaporation. The light property of displays is influenced by surfacemorphologies of films. Thus, the interval between two adjacentnanoparticles and the morphology of the nanoparticles are importantfactor for films in displays.

FIG. 3A and FIG. 3B are sectional views showing vertical and lateralelectric fields of the LCD, respectively. In a conventional LCD device,a continuous-grain structure of IZO or ITO exerts an influence on avertical electrical field and a lateral electrical field formed adjacentto two electrodes of the color filter and TFT substrates 110 and 130.Namely, liquid crystal molecules are aligned throughout the LCDaccording to the features of ITO or IZO films.

In case of films of nanoparticles such as nanotubes and nanowires, adensity of the nanoparticles and a uniformity of an interval between twoadjacent nanoparticles are important factors in achieving uniformvertical and lateral electric fields as shown in FIG. 3A and FIG. 3B.

When forming an electrode in a form of a transparent conductive filmincluding nanoparticles, vertical and lateral electric fields applied tothe liquid crystal molecules may be non-uniform when the density of thetransparent conductive film is under a certain range. The non-uniformityof the electric fields means there may be non-uniformity of nanoparticledensity as well as non-uniformity of intervals between adjacentnanoparticles. Thus, when a transparent conductive film has less acertain density value, even though the value fulfills the basicelectrical and optical requirements of an electrode such as a commonelectrode, the interval and the density in the film may cause ‘microMura.’ The micro Mura affects the overall contrast ratio of the displayapparatus.

When using a transparent conductive film including nanoparticles as anelectrode in an electrophoretic display, a contrast ratio of theelectrophoretic display decreases with a thin film density under acertain range. In case of a low density thin film, an interval betweenadjacent nanoparticles may be greater than a size of an electrophoreticmicrocapsule of about 20 μm to about 40 μm. The interval causes apartial charge of the electrophoretic microcapsules in both white andblack states when an electric field is applied to the electrophoreticmicrocapsules. Hence, an overall contrast ratio of the electrophoreticdisplay decreases. In order to support a required contrast ratio, anadditional voltage is required, resulting in greater power consumption.

On the contrary, when the transparent conductive film includingnanoparticles has a density over a certain range, transmittancedecreases and the contrast ratio decreases, thereby causingdeterioration in image quality.

Therefore, it is important to determine the density of nanoparticlessuch as nanowires and carbon nanotubes when using the transparentconductive film as the electrode, to determine a balance between opticaland electrical properties while maintaining transmittance or sheetresistance adequately. The transparent conductive film may satisfy thecondition of sheet resistance of about 1000 ohms per square or less andtransmittance of about 70% or more in order to be used as the electrode.When the transparent conductive film satisfies the sheet resistance andthe transmittance conditions, an additional voltage to drive thenanoparticle electrode may not be required even when the transparentconductive film has a low density. In addition, a significant reductionin display brightness may not occur even when the transparent conductivefilm has a high density.

Metal nanowires, metal oxide nanoparticles, carbon nanotubes, and so onmay be used as nanoparticles that may form a transparent conductivefilm.

Metal nanowires have an average diameter of about 20 nm and have nobundling effect, so that each nanowire is present individually. Carbonnanotubes are carbon-based nanoparticles and composed entirely ofcarbon. Most carbon nanotubes have a cylindrical shape. When the carbonnanotubes have a spherical shape or an ellipsoid shape, they arereferred to as fullerenes.

Carbon nanotubes are classified into single-walled nanotubes,double-walled nanotubes, and multi-walled nanotubes, which are adjustedto obtain various electrical or optical properties. Ink-based nanotubesare typically produced by technology using solvents or surfactants, andmay be coated over wide regions using a non-vacuum apparatus, such as aspray, roll, slit, spin coater, inkjet, etc.

Carbon nanotubes exist in a form of bundles of single carbon nanotubes.The single carbon nanotubes include both a metallic type and asemiconductor type in which its conductivity is degraded. The metallictype carbon nanotubes and the semiconductor type carbon nanotubes existin the single carbon nanotubes by a ratio of 1:3. The bundles typicallyhave diameters in the range between about 5 nm and about 100 nm. Whenusing the carbon nanotubes as the electrode in the form of a transparentconductive film for a display apparatus, a density of the carbonnanotubes depends on a coating process. In addition, the density of thecarbon nanotubes for the electrode should be three or five times that ofmetal nanowires in order to make the carbon nanotubes have a similarconductivity to that of nanowires.

Metal oxide nanomaterials are referred to as nanoparticles and includemetal oxide nanoparticles such as TiO₂, RuO₂, SrRuO₂, and so on. Forexample, sol-gel thin film based on ITO and Indium-doped ZnO may be usedas the electrode in the form of the transparent conductive film. The ITOmetal oxide nanoparticles in the transparent conductive film have adensity of about 50 particles to about 80 particles per squaremicrometer, which is ten times the density of nanowires.

The nanoparticles may be used to form a transparent conductive film. Thenanoparticles may be prepared by a non-vacuum process to have a metallicproperty, such as conductivity over 1 S/cm. However, since thenanoparticles used to form the transparent conductive film have variousconductivities according to the kind of the nanoparticles, adistribution of the nanoparticles in terms of a density should becontrolled to reach the sheet resistance and the transmittance suitablefor the electrode of a display apparatus.

Hereinafter, a density range of nanoparticles used for the transparentconductive film as the electrode for the display apparatus accordingexemplary embodiments of the present invention will be described.

Table 2 shows sheet resistance and transmittance in relation to adensity of metal nanowires at a cell gap of 5 μm. Metal nanowires wereformed in a form of transparent conductive film as an electrode for anLCD, and then the sheet resistance and the transmittance were measured.The density value in Table 2 refers to the average number of metalnanowires per 5×5 square micrometers. Numbers in parentheses refer torough numbers of nanowires per square micrometer.

TABLE 2 sheet resistance transmittance Density (Ω/□) (%) 0 (1 or less) —92 5~10 (1~2) 150~500 85~92 10~25 (2~5)  75~150 80~85 25~40 (5~8) 25~7580~75 50 (10 or more) 10~30 75 or less

Referring to Table 2, as the density of metal nanowires increases, thesheet resistance and the transmittance decrease. As the density of metalnanowires decreases, the sheet resistance and the transmittance alsoincrease.

When the density of the metal nanowires is zero in Table 2, it meansthat one particle or no particle exists per square micrometer. In thiscase, although the sheet resistance is not shown in the Table 2 when thedensity of the metal nanowires is zero, the sheet resistance is toohigh, for example, mega ohms, to drive the LCD, so that the metalnanowires are not suitable for the electrode.

Similarly, in case that the density of the metal nanowires is 40particles or more per 25 square micrometers, for example 50 particlesper square micrometer as shown in Table 2, the transmittance is of 75%or less. In this case, although the sheet resistance is low enough todrive liquid crystal molecules, the brightness of the display apparatusbecomes too low due to the transmittance of 75% or less.

FIG. 4 is a graph showing transmittance of metal nanowires in relationto an applied gray scale voltage at a cell gap of 5 μm.

In general, in order to use a transparent conductive film as anelectrode of an LCD, the transmittance of the transparent conductivefilm should be over a certain level when no voltage (0V) is applied tokeep the gray scale, and should be reduced enough when a low voltage isapplied to display a black color.

Referring to FIG. 4, as a density of metal nanowires increases,reduction in transmittance occurs at a low gray scale voltage. However,the transmittance is not reduced enough to display the black color whenan electrode has too low density of the metal nanowires, for example,0˜1 particles per square micrometer. Therefore, the metal nanowires inlow density are not suitable for the transparent conductive film becausethe black color is difficult to obtain.

On the other hand, the electrode having a very high density of the metalnanowires (referred to as ‘out of range’ in FIG. 4), for example, 10particles or more of the metal nanowires per square micrometer, is ableto produce the black color in high quality since the transmittance isreduced enough to display the black color even when a low gray scalevoltage is applied. However, when no voltage (0V) is applied, thetransmittance is too low to display an image, so that the electrodehaving the very high density of metal nanowires is not also suitable forthe display apparatus.

Table 3 shows sheet resistance and transmittance in relation to adensity of metal oxide nanoparticles at a cell gap of 5 μm. The metaloxide nanoparticles were formed in a form of transparent conductive filmas an electrode for an LCD, and then the sheet resistance and thetransmittance were measured. The density value in Table 3 refers toaverage number of the metal oxide nanoparticles per 5×5 squaremicrometers. Numbers in parentheses refer to rough numbers of the metaloxide nanoparticles per square micrometer.

TABLE 3 sheet resistance transmittance Density (Ω/□) (%)  500~1300(100~260)  600~1000 89~84 1300~2250 (260~450) 600~400 84~79 2250~3000(450~600) 400~250 79~75

Referring to Table 3, as the density of metal oxide nanoparticlesincreases, the sheet resistance and the transmittance decrease. Thesheet resistance and the transmittance increase as the density of metaloxide nanoparticles decreases.

When the transparent conductive film has a density of 500 metal oxidenanoparticles or less per square micrometer, the transparent conductivefilm has the transmittance of about 89% or more, resulting in hightransmittance. In this case, however, since the transparent conductivefilm has a kilo-ohm scale sheet resistance, i.e. 1000 or more, thetransparent conductive film is difficult to use as the electrode for thedisplay apparatus.

On the contrary, in case of the transparent conductive film having thedensity of 3000 metal oxide nanoparticles or more per square micrometer,the transparent conductive film has the transmittance of about 75% orless, so that the transparent conductive film is not appropriate to beused as an electrode for the display apparatus, even though the sheetresistance of the transparent conductive film is sufficiently low.

FIG. 5 is a graph showing transmittance of metal oxide nanoparticles inrelation to an applied gray scale voltage at a cell gap of 5 μm.

Similar to the metal nanowires, in order to use the metal oxidenanoparticles as the electrode of the display apparatus, thetransmittance of the transparent conductive film should be over acertain level even when no voltage (0V) is applied to keep the grayscale, and should be reduced enough even when a low voltage is appliedto display the black color.

Referring to FIG. 5, when the transparent conductive film has a very lowdensity of metal oxide nanoparticles, for example, 100 particles or lessper square micrometer, the transmittance of the transparent conductivefilm does not decrease enough even though a voltage is applied thereto.This means that displaying the black color is difficult at very lowdensity, so that the transparent conductive film having the very lowdensity is not appropriate for the electrode of the display apparatus.

On the other hand, the transparent conductive film having a very highdensity of metal oxide nanoparticles, for example, 600 particles or moreof metal oxide nanoparticles per square micrometer, is able to displaythe black color even though a small gray scale voltage is appliedthereto. However, the transparent conductive film having the very lowdensity of the metal oxide nanoparticles is not appropriate for theelectrode of the display apparatus since transmittance is low when novoltage is applied thereto.

Table 4 shows sheet resistance and transmittance in relation to adensity of carbon nanotubes at a cell gap of 5 μm. The carbon nanotubeswere formed in a form of transparent conductive film as an electrode foran LCD, and then the sheet resistance and the transmittance weremeasured. The density value in Table 4 refers to average number ofcarbon nanotubes per 5×5 square micrometers. Numbers in parenthesesrefer to rough numbers of carbon nanotubes per square micrometer.

TABLE 4 sheet resistance transmittance Density (Ω/□) (%) 20~60 (4~12)400~800 88~83 60~100 (12~20) 250~400 83~78 100~150 (20~30) 150~250 78~73

Referring to Table 4, as the density of carbon nanotubes increases, thesheet resistance and the transmittance decrease. As the density ofcarbon nanotubes decreases, the sheet resistance and the transmittanceincrease.

The transparent conductive film to which the carbon nanotubes areapplied, similar to the metal nanowires and the metal oxidenanoparticles, has a transmittance of about 88% or more at a low density(under 4 particles or less per square micrometer). But since thetransparent conductive film has a high sheet resistance of kilo-ohm, thetransparent conductive film is difficult to use for the electrode of thedisplay apparatus.

On the contrary, when the transparent conductive film has a high densityof carbon nanotubes, for example 30 particles or more per squaremicrometer, the transparent conductive film having the high density isnot appropriate for the electrode of the display apparatus since thetransparent conductive film has a low transmittance of about 73% or lesseven though its sheet resistance is sufficiently low.

FIG. 6 is a graph showing transmittance of carbon nanotubes in relationto an applied gray scale voltage at a cell gap of 5 μm.

As shown in FIG. 6, as the density of the transparent conductive filmincreases, the transmittance may be lowered under the low gray scalevoltage similarly to the metal nanowires or the metal oxidenanoparticles. However, the transmittance of a transparent conductivefilm may be kept over a certain level in order to maintain the grayscale when no voltage is applied thereto.

In the present example, the transmittance does not decrease enough todisplay the black color even though a voltage is applied while thetransparent conductive film has a very low density of carbon nanotubes,for example, 4 particles or less per square micrometer. The transparentconductive film is not appropriate for the electrode of the displayapparatus.

On the other hand, the transparent conductive film having the highdensity of carbon nanotubes, for example, 30 particles or more of carbonnanotubes per square micrometer, is able to display the black color eventhough the small gray scale voltage is applied thereto. However, thetransparent conductive film is not appropriate for the electrode ofdisplay apparatus since transmittance is low when no voltage is appliedthereto.

As described above, the nanoparticles for the transparent conductivefilm used as the electrode for the display apparatus have a densityvalue within a certain range.

Table 5 shows the density range and the transmittance of nanoparticles.

TABLE 5 conductivity density at a cell gap of 5 μm (S/cm) (D: diameter,L: length) metal 1000~2000 5~40 particles (D: 20 nm, L: 1 μm) nanowiresmetal oxide <100 500~3000 particles (D: 20 nm) nanoparticles carbon 400~1500 20~150 particles (D: 20 nm, L: 1 μm) nanotubes

Table 6 shows an adequate range of the density of nanoparticles at acell gap of about 4 to about 6 μm according to the exemplary embodimentsdescribed above. The density value refers to an average number ofnanoparticles per 5×5 square micrometers.

TABLE 6 cell gap 4 μm 5 μm 6 μm metal nanowires 4~20 5~40 6~40 metaloxide 400~2400 500~3000 600~3000 nanoparticles carbon nanotubes 12~14720~150 20~157

The distribution of the density for the transparent conductive filmdepends on the conductivity of the nanomaterials as shown in Table 5.The conductivity of the transparent conductive film is related to thesheet resistance necessary to apply electrical signals to liquid crystalmolecules at a specific cell gap. Moreover, in order to maintain aconstant level of electrical signals applied to the liquid crystalmolecules while the cell gap increases, the transparent conductive filmshould have sufficient conductivity.

The density of the transparent conductive film is about ten or less toabout several thousands to satisfy the conductivity and the sheetresistance requirements for the electrode of the display apparatus. Thetransparent conductive film having the appropriate density range mayalso be used in various fields such as flat panel displays, solar cells,radio-frequency identification (RFID), and so on.

In the transparent conductive film having the density described in Table6, an interval between two adjacent nanoparticles may cause Muradefects, for example, non-uniformity of sheet resistance ortransmittance.

When an average interval between adjacent nanowires in the transparentconductive film is less than a cell gap (e.g. about 5 μm) or more closerto domains of liquid crystal molecules, which are normally of about 0.3to about 1 μm, Mura may be reduced or be totally removed. In this case,the transmittance of the transparent conductive film is about 80% toabout 85% at a wavelength of about 550 nm.

When the interval becomes smaller than about 0.2 μm, the transmittanceis below 50% even though Mura does not occur, thereby reducing thebrightness of the display apparatus below working range.

When the interval is higher than the cell gap of about 5 μm, Mura occurseven though the transmittance is above 85%. The Mura results fromnon-uniformity of the nanoparticles. When the interval between adjacentnanowires is about 30 μm, the nanoparticles are not appropriate for thedisplay apparatus since Mura may occur even when the transmittance isabout 90%.

Table 7 shows optimized intervals between two adjacent nanoparticlesaccording to the kind of nanomaterial used in the transparent conductivefilm.

TABLE 7 cell gap 4 μm 5 μm 6 μm metal nanowires   1~0.2   1~0.2   1~0.2metal oxide 0.01~1.6 × 10⁻³ 0.01~2 × 10⁻³ 0.01~2.3 × 10⁻³ nanoparticlescarbon nanotubes 0.33~0.027 0.33~0.033 0.33~0.038

Referring to Table 7, the transparent conductive film was prepared byspin-coating using high-concentrated ink, resulting in a density ofnanowires of about 5 to about 25 particles per square micrometer, aninterval between adjacent nanowires of about 1 to about 5 μm, a sheetresistance of about 20 Ω/□, and a transmittance of 80%. As a result,Mura was reduced in the transparent conductive film.

Further, the conductivity is maintained with a higher density of 30nanowire particles/μm2, but the transmittance is reduced drasticallyunder about 73%. The transparent conductive film having thetransmittance under about 73% is not appropriate for the displayapparatus since the transmittance of about 73% is lower than thatrequired for the display apparatus, which may thereby cause a decreasein the overall image quality of the display apparatus.

The transparent conductive nanomaterials according to the exemplaryembodiments of the present invention may be used for the electrode byitself, but if necessary, a nanocomplex in which the nanoparticles areconnected to each other by the binder may be used for the transparentconductive film.

The carbon nanotubes or the metal nanowires among transparent conductivenanomaterials have an aspect ratio of 10 or more. Hence, the transparentconductive film of the carbon nanotubes or the metal nanowires may havethe appropriate conductivity depending on whether the network structureexists in each nanoparticle.

However, although the electrode of the carbon nanotubes or the metalnanowires appears to be formed uniformly when seen with a macroscopicview, the electrode has substantial non-uniformity when seen with amicroscopic view. For example, a silver nanowire film has uniformity interms of arrangements of the nanoparticles with a macroscopic view.However, with a microscopic view using a scanning electron microscope(SEM), the silver nanowire film has intervals between silver nanowiresthat are non-uniform. The interval non-uniformity may be observed overthe transparent conductive film with a microscope.

For the observance of the interval non-uniformity, after a transparentconductive film is formed on substrates with high-density silvernanowires, a photosensitive or thermosetting resin is coated over asurface of the transparent conductive film in order to improve adhesionbetween the transparent conductive film and an alignment layer. Thealignment layer including polyimide is coated over the resin, and thenthe alignment layer is rubbed. The substrates are assembled to eachother by using a photosensitive sealant surrounding liquid crystalmolecules interposed between the substrates. After the sealant is curedby light, polarizers are attached on the substrates, respectively tocomplete the liquid crystal display. When observing the LCD with amicroscope, the non-uniformity of the image is found, which may becaused by the non-uniform alignment of the liquid crystal molecules.

The non-uniformity of the liquid crystal molecule alignment shows thatmicroscale non-uniformity may exist even in a high-density nanowire filmand may form texture that exerts influence on visibility of the LCD.Hence, reducing microscale non-uniformity of nanomaterials may berequired, in order to use the nanomaterials in a transparent conductivefilm, especially for a high resolution LCD.

Therefore, in an exemplary embodiment of the present invention, thenanocomplex based on the transparent conductive nanomaterials may beused as the material for the electrode of the display apparatus in orderto reduce the non-uniformity of the nanomaterials.

FIG. 7A is a photograph showing gold-silver nanoparticles as arepresentative nanomaterial. FIG. 7B is a photograph showing agold-silver nanocomplex comprising gold and silver nanoparticles.

As shown in FIG. 7A and FIG. 7B, the nanocomplex has a high aspectratio. Due to the high aspect ratio, the nanocomplex also has highflexibility as well as small a range of intervals between nanowirescompared with a range of intervals between individual nanoparticles. Thesmall range of the intervals results from the network structure ofnanoparticles that are randomly connected to each other. In addition,since the interval between adjacent nanocomplexes is less than theinterval of the individual nanowires, which are in the form ofparticles, the nanocomplex of nanowires has haziness of about 0.1% withtransmittance of 90% or more. On the contrary, the nanowires in the formof individual particles have haziness of at least 1%.

The nanoparticles may be formed by using various schemes. For example, atechnology based on biological templates using a nucleation process foruniform growth of metal nanowires may be used. The nanowires in thenucleation process are produced while a conductive precursor or a seedis converted to conductive nanoparticles that bind to the biologicaltemplates. The seed may be Ni, Cu, Pd, Co, Pt, Ru, Ag, Co alloys, or Nialloys. Metals, metal alloys, and metal oxides may be plated on the seedand may include Cu, Au, Ag, Ni, Pd, Co, Pt, Ru, W, Cr, Mo, Ag, Co alloys(e.g., CoPt), Ni alloys, Fe alloys (e.g., FePt), TiO2, Co3O4, Cu2O,HfO2, ZnO, vanadium oxides, indium oxide, aluminum oxide, indium tinoxide, nickel oxide, copper oxide, tin oxide, tantalum oxide, niobiumoxide, vanadium oxide or zirconium oxide, but the present invention isnot limited thereto. The biological templates include proteins,peptides, phages, bacteria, viruses, and the like. The technology basedon the biological templates is also referred to as ‘mineralization’ or‘plating’. For example, a metal precursor (e.g., a metal salt) can beconverted to an elemental metal in the presence of a reducing agent. Asa result, the elemental metal binds to the biological templates andgrows into a continuous metallic layer.

The silver nanowires prepared by the above technology have a uniformdiameter of about 20 nm and a length of several microns.

A second technology is based on a polyol process for a mass productionof silver nanowires with a uniform diameter. The second technologyinvolves the reduction of silver nitrate by ethylene glycol in thepresence of polyvinylpyrrolidone (PVP). When the silver nitrate isreduced in the presence of seeds (Pt or Ag particles of a fewnanometers), the silver nanoparticles with a bimodal size distributionare generated in a reaction mixture via heterogeneous and homogeneousnucleation processes, respectively. With the second technology, thesilver nanowires, each of which having a diameter of about 30 to about60 nm and a length of about 1 to about 50 μm, are obtained. Namely, byusing the second technology, the mass production of the silvernanowires, each of which have a diameter of about 15 to about 25 nm anda length of tens of micrometers, may be available. In addition, in thepresence of gemini surfactant 1,3-bis(cetyldimethylammonium) propanedibromide (16-3-16), the nanowires, each of which having an uniformaspect ratio above about 2000, may be obtained.

Various technologies may be used to produce nanotubes in sizeablequantities, such as arc discharge, laser ablation, high pressure carbonmonoxide (HiPCO), and CVD, or the like. These processes in general takeplace in a vacuum or with process gases. Large quantities of nanotubescan be synthesized using a catalysis process and a continuous growthprocess. For synthesis of the carbon nanotubes, some kinds of catalystscan be used. Among the above-described technologies, laser ablation andCVD have shown to have high yield and good control performance on thediameter of the nanotubes.

Hereinafter, a method of manufacturing the display apparatus accordingto an exemplary embodiment of the present invention will be describedwith reference to FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, and FIG. 8E.

FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, and FIG. 8E are sectional viewssequentially showing a method of manufacturing a display apparatusaccording to an exemplary embodiment of the present invention.

As shown in FIG. 8A, a first insulating substrate 101 is prepared. Then,a gate electrode 113 and a gate line 111 are formed on the firstinsulating substrate 101. The first insulating substrate 101 may be madeof glass or plastic. The gate electrode 113 and the gate line 111 may beformed by depositing a first conductive layer on a whole surface of thesubstrate 101 and patterning the first conductive layer through aphotolithography process.

Next, as shown in FIG. 8B, a gate insulating layer 115, an amorphoussilicon layer, and an n+ amorphous silicon layer are sequentiallydeposited on the whole surface of the first insulating substrate 101.The amorphous silicon layer and the n+ amorphous silicon layer areselectively patterned through a photolithography process to form anactive layer 117 and an ohmic contact layer 119 that ohmic-contacts asource electrode 122 and a drain electrode 123, which are subsequentlyformed.

Then, as shown in FIG. 8C, a second conductive layer is formed on thewhole surface of the first insulating substrate 101 having the activelayer 117 and the ohmic contact layer 119. The second conductive layeris selectively patterned through a photolithography process to form thesource electrode 121 and the drain electrode 123. The source electrode122 serves as a portion of the data line 117 crossing the gate line 111to define a pixel area.

The active layer 117, the ohmic contact layer 119, and the source anddrain electrodes 122 and 123 may be formed through the two-stepphotolithography process described above, but the present invention isnot limited thereto. For example, the ohmic contact layer 119 and thesource and drain electrodes 122 and 123 may be formed through a singlephotolithography process with a refractive mask or a half-tone mask.

As shown in FIG. 8D, a protection layer 125 is deposited on the wholesurface of the first insulating substrate 101 and the protection layer125 is patterned through a photolithography process. During patterning,a contact hole 129 is formed in the protection layer 125 to expose aportion of the drain electrode 123.

As shown in FIG. 8E, a transparent conductive material is formed on thewhole surface of the first insulating substrate 101. The transparentconductive material is selectively patterned through a photolithographyprocess, so that a pixel electrode 127 is formed. The pixel electrode127 is electrically connected to the drain electrode 123 through thecontact hole 129.

The pixel electrode 127 is formed with transparent nanomaterialsincluding nanoparticles such as metal nanowires, metal oxidenanoparticles, carbon nanotubes, and so on. The pixel electrode 127 maybe formed through a spin coating scheme, a web coating scheme, a gravureprinting scheme, and so on. In addition, the transparent nanomaterialsmay be formed by using a non-vacuum apparatus such as a spray, roll,slit, spin coater, inkjet, etc., at a low temperature atmosphere.

A second insulating substrate is prepared (not shown). A color filter isformed on the second insulating substrate. Then, a common electrode isformed on the color filter.

Although not shown in drawings, the first insulating substrate and thesecond insulating substrate face each other, and a liquid crystal layeris formed between the first insulating substrate and the secondinsulating substrate, thereby completing the display apparatus.

In the present exemplary embodiment, the transparent conductive film hasbeen described for the LCD, but the present invention is not limitedthereto. For example, the transparent conductive film may be used tovarious display panels such as an OLED, a PDP, and an electrophoreticdisplay panel.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A display apparatus, comprising: a first substrate comprising aplurality of pixels; a first electrode arranged on the first substrate;a second substrate facing the first substrate; and a second electrodearranged on the second substrate and spaced apart from the firstelectrode, the second electrode to form an electric field in cooperationwith the first electrode, wherein at least one of the first electrodeand the second electrode comprises a transparent conductive nanomaterialhaving a transmittance of no less than 73% to no more than 100%, and asheet resistance of 0 ohms per square to 100 ohms per square.
 2. Thedisplay apparatus of claim 1, wherein the transparent conductivenanomaterial comprises one of metal nanowires, metal oxidenanoparticles, and carbon nanotubes.
 3. The display apparatus of claim2, wherein a cell gap between the first substrate and the secondsubstrate is in the range of 4 μm to 6 μm.
 4. The display apparatus ofclaim 3, wherein the transparent conductive material comprises metalnanowires, and a density of the metal nanowires is in the range of 4particles to 40 particles per 5×5 square micrometers.
 5. The displayapparatus of claim 4, wherein a maximum interval between two adjacentmetal nanowires among the metal nanowires is 0.2 μm to 1 μm.
 6. Thedisplay apparatus of claim 3, wherein the transparent conductivematerial comprises metal oxide nanoparticles, and a density of the metaloxide nanoparticles is in the range of 400 particles to 3000 particlesper 5×5 square micrometers.
 7. The display apparatus of claim 6, whereina maximum interval between two adjacent metal oxide nanoparticles amongthe metal oxide nanoparticles is 1.6×10⁻³ μm to 0.01 μm.
 8. The displayapparatus of claim 3, wherein the transparent conductive materialcomprises carbon nanotubes, and a density of the carbon nanotubes is inthe range 4 particles to 150 particles per 5×5 square micrometers. 9.The display apparatus of claim 8, wherein a maximum interval between twoadjacent carbon nanotubes among the carbon nanotubes is 0.027 μm to 0.33μm.
 10. The display apparatus of claim 1, wherein the transparentconductive nanomaterial comprises a nanoparticle-based nanocomplex. 11.The display apparatus of claim 10, wherein the nanocomplex comprisessilver particles and gold particles.
 12. The display apparatus of claim11, wherein the silver particles and the gold particles are in ananowire form.
 13. The display apparatus of claim 1, further comprisinga liquid crystal layer disposed between the first substrate and thesecond substrate.
 14. A thin film transistor substrate, comprising: asubstrate; a thin film transistor arranged on the substrate; and atransparent conductive electrode connected to the thin film transistor,the transparent conductive electrode comprising metal nanowires having adensity in the range of 4 particles to 40 particles per 5×5 squaremicrometers.
 15. The thin film transistor substrate of claim 14, whereina maximum interval between two adjacent metal nanowires among the metalnanowires is 0.2 μm to 1 μm.
 16. The thin film transistor substrate ofclaim 14, wherein the transparent conductive electrode comprises ananoparticle-based nanocomplex.
 17. The thin film transistor substrateof claim 16, wherein the nanocomplex comprises silver particles and goldparticles.
 18. The thin film transistor substrate of claim 17, whereinthe silver particles and the gold particles are in nanowire form.
 19. Athin film transistor substrate, comprising: a substrate; a thin filmtransistor arranged on the a substrate; and a transparent conductiveelectrode connected to the thin film transistor, the transparentconductive electrode comprising metal oxide nanoparticles having adensity in the range of 400 particles to 3000 particles per 5×5 squaremicrometers.
 20. The thin film transistor substrate of claim 19, whereina maximum interval between two adjacent metal oxide nanoparticles amongthe metal oxide nanoparticles is 1.6×10⁻³ μm to 0.01 μm.
 21. A thin filmtransistor substrate, comprising: a substrate; a thin film transistorarranged on the substrate; and a transparent conductive electrodeconnected to the thin film transistor, the transparent conductiveelectrode comprising carbon nanotubes having a density in the range of 4particles to 150 particles per 5×5 square micrometers.
 22. The thin filmtransistor substrate of claim 21, wherein a maximum interval between twoadjacent carbon nanotubes among the nanotubes is 0.027 μm to 0.33 μm.